Solution-based processing has attracted huge attentions for its advantages of large-scale and cost-effective fabrication of various applications. However, despite of remarkable advances in organic semiconducting materials, relatively low electrical properties of organic materials limits its practical application.
We are designing novel all-inorganic inks comprising of soluble inorganic precursors or nanocrystals for the fabrication of thin films with tailored composition, structure, and properties. The materials of interest include semiconducting metal chalcogenides and phosphorus, metallic Ag, Cu, and 2D MXenes, applied to thermoelectric,electronic, optoelectronic, energy, and flexible devices.
Nano 3D printing is a technology included in the world’s 50 innovation technology in Frost & Sullivan 2017 report. Nano 3D printing could play an important role in micro-, and nano- scale 3D fabrication such as MEMS. We are developing novel nanocrystal inks and processing for designing nano-scale 3D architecture with tailored structure and properties. We design stimuli-responsive, assembly-programmable nanocrystal building blocks through chemical engineering of surfaces and interfaces. To this end, we are exploring fundamental chemistry and physics governing the assembly of nanocrystal building blocks under various external stimuli. Architectured 3D matters will find potential applications of 3D electronics, sensors, catalysts as well as platform of lithography.
Extrusion-based additive manufacturing process is the most basic form of 3D printing which relies solely on the rheological property of the ink (Fig. a). These processes include dispenser printing, and robocasting. In dispenser printing, ink is dispensed out of the micrometer nozzle directly to the substrate by pneumatic control. Since there is no complex constituent that is required for this printing process, the processing parameter itself is relatively simple compared to other manufacturing techniques. For dispensing printing method, the ink must exhibit non-Newtonian and viscoelastic behavior. The inks must be tailored to simultaneously have shear-thinning behavior, which facilitates reliable extrusion flow through the nozzle to prevent clogging, and high elasticity to avoid the collapse of the 3D structure (Fig. b).
As a model application of this processing, we focus on the development of viscoelastic inks containing thermoelectric particles. Thermoelectric inks are often paired with polymer-based organic binder to give rise to viscoelastic behavior. Thermoelectric performance of these composite inks with polymer matrix is strictly limited due to low power factors. Thermoelectric inks with organic binder suffer from low electrical property due to its intrinsically insulating property. We are replacing the electrically insulating polymer matrix with molecular chalcogenidometallate-based (ChaM) ions, anions containing metal atoms ligated with chalcogens (Fig. c).
Residual solvent inside the printed structures were dried and sintered to consolidate Bi2Te3 particles into one compound. During heat treatment, Sb2Te3 ChaM ions promoted sintering process without the introduction of secondary phases (Fig. d). The structural transformation from molecular ChaM ions to crystalline phases lead to a formation of mechanically robust product as well as increased ZT values (Fig. e).
